Contents

Introduction

On this page I will show how to use Erlang on RPi to put a sensor value from Gertboard on the web using an interface written in Forth and how to put a picture from Pi Camera on the web.

A Web Server

Erlang is a perfect language to use for programming a web server, and that is exactly what Loïc Hoguin did. [2] Cowboy is one of the most scalable web servers there is and it's also small so it fits nicely in a RPi. I was thinking of using it to display the kW load from my Atlast Forth interface to Gertboard. Here is how I did it, first install the cowboy web server and test run it:

External interfaces - Gertboard

Erlang processes communicate with the outside world using the same message passing mechanism as used between Erlang processes. This mechanism is used for communication with the host operating system and for interaction with programs written in other languages. If required for reasons of efficiency, a special version of this concept allows e.g. C programs to be directly linked into the Erlang runtime system. The easy way is good enough so just open a port to Atlast Forth in the file examples/hello_world/src/toppage_handler.erl. Replace toppage_handler.erl with the following:

Ok, now we just have to call start.sh again ( sudo is needed by the Gertboard driver, not by Cowboy, and for pure simplicity I take another shortcut and starts the Cowboy with sudo rights. Don't do this in a production environment).

sudo ./start.sh

Point the browser to 192.168.0.178:8080 or whatever ip address you have on your RPi. After a few seconds the W load should show up. I stop here, this is not the right forum to discuss how to make a web gui, lots of other sites can help with that. Have fun :)

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.

External Interfaces - Pi Camera

If you have a PiCamera and wants to have a picture on a web page. The cool thing with this is that it's in real time, it is not a file that is served but a new picture is taken when you do the web call. Check that /opt/vc/bin/raspistill works on your system, then replace the code in examples/hello_world/src/toppage_handler.erl with the following: (Then follow the Gertboard example for compiling and running).

Note: The suggested way to call the port directly from examples/hello_world/src/toppage_handler.erl is just to make it easy. In a real website with thousands of users you should open the port from a separate process and let it run and serve more than one web call.